Conventionally, a spark plug has been used for an internal combustion engine. For example, such a spark plug includes an insulator having a through-hole, and a metallic shell disposed around the insulator in the radial direction. When the insulator is exposed to a combustion gas, carbon may be adhered to the surface of the insulator. Such carbon may cause a problem. For example, unintended discharge may occur inside the metallic shell through the carbon. As a technique for suppressing such a problem, a technique has been proposed in which the area of a space formed by the surface of a leg portion of the insulator and the inner wall surface of the metallic shell is reduced to prevent entry of the combustion gas, thereby improving anti-fouling characteristics of the leg portion of the insulator.